1. Field of the Invention
This invention relates to a light conducting plate for a surface light source device, which light conducting plate enables a high luminance to be obtained with unlimited uniformity over the entire surface thereof, and to a method for the manufacture of the light conducting plate.
2. Description of the Prior Art
Liquid crystal displays have made spectacular advances in recent years and are now utilized in a wide range of fields including the office automation sector, television receivers, automotive related sectors, the communications equipment sector, and advertising displays. Since liquid crystal is not self-illuminating, the display was initially read using reflected light but because of the very poor visibility by this method, backlit display devices made visible using transmitted light have become the mainstream and are in wide use. As the light source of the transmitted light in such a backlit display, there is used a so-called back light. Back lighting includes the under lighting system in which a fluorescent tube is disposed below a diffusion panel as a linear light source and various techniques are implemented in an attempt to secure a uniform surface light source, and the edge lighting system in which a fluorescent tube is disposed at the edge of a light conducting plate as a linear light source and various techniques are implemented for obtaining a surface light source by uniformly projecting on and emitting from the upper surface of the light conducting plate the light received through the edge thereof.
In general, the under lighting system is bright but has a thick light conducting plate, while the edge lighting system is characterized in being darker but having a thinner light conducting plate than the under lighting system. The system to be used is selected according to the application.
Further, a reflecting layer is formed on one surface of the light conducting plate for obtaining uniform luminance of the light-emitting surface and it is well known to form the reflecting layer by the method of printing a dot pattern using ink having light reflection and diffusion property. By this printing method, however, since a part of the light transmitting through the light conducting plate is absorbed by the ink itself, the light utilization efficiency is decreased by a corresponding amount. Moreover, an ink drying step is required after the printing. In addition, the product quality is hindered and the value of the product diminished should, for example, minute dust particles adhere to the undried ink during printing or drying or should a part of the ink peel off, and there is therefore the drawback of extremely low yield.
Methods were thus found for cutting multiple grooves in the light conducting plate by subjecting the surface of the light conducting plate formed with the reflecting layer to machine processing or molding processing, and various configurations have been proposed.
In the prior art light conducting plate for a surface light source device cut with such grooves, however, if the grooves are formed in the direction perpendicular to the direction of light transmission, while some degree of luminance can be obtained at positions where the distance from the linear light source to the groove is short, only very low luminance can be obtained at positions where the distance from the linear light source to the groove is long, because the reflecting surfaces of the grooves near the linear light source hinder the transmission of the light, with the result that uneven luminance arises over the entire surface of the light conducting plate. When the grooves are cut to pass in the direction of light transmission, moreover, the luminance is extremely low because the grooves scatter almost no light in the direction of the light emitting surface.
Thus while technology has been proposed for a light conducting plate formed with a light scattering layer by cutting grooves, the actual situation is that it has not been reduced to a practical product because of extremely low luminance and the occurrence of uneven luminance.
As a result, a need is felt for a light conducting plate for a surface light source device which does not employ a method of printing a dot pattern using ink, which is further capable of unlimited uniformity over the entire surface of the light conducting plate, and which is further able to provide luminance equal to or higher than a light conducting plate printed with a dot pattern.
This invention was accomplished in light of the aforesaid earlier defects and also for responding to a long felt need, and has as its object to provide a light conducting plate for a surface light source device which, though not printed with a dot pattern, has a luminance equal to or higher than a light conducting plate printed with a dot pattern, and a method for the manufacture thereof.